Systems and methods for forming a layer onto a surface of a solid substrate and products formed thereby

ABSTRACT

A method for forming a vehicular brake rotor involving loading a shaped metal substrate with a mixture of metal alloying components and ceramic particles in a dieheating the contents of the die while applying pressure to melt at least one of the metal components of the alloying mixture whereby to densify the contents of the die and form a ceramic particle-containing metal matrix composite coating on the metallic substrate; and cooling the resulting coated product.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/822,023, filed Nov. 24, 2017, which in turn is a divisional of U.S.patent application Ser. No. 15/357,730, filed Nov. 21, 2016, now U.S.Pat. No. 9,933,031, granted Apr. 3, 2018, which claims priority fromU.S. Provisional Application Ser. No. 62/258,448, filed Nov. 21, 2015,and from U.S. Provisional Application Ser. No. 62/265,765, filed Dec.10, 2015, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the formation of substantially porefree wear resistant metal ceramic particulate composites on metalsubstrates, systems for forming such composite layers, and productsmade. The invention has particular utility in connection with vehicularbrake rotors and will be described in connection with such utility,although other utilities are contemplated.

BACKGROUND OF THE INVENTION

Land vehicles including cars, trucks, trains, and mass transit systemsuse disc brakes to slow and/or stop the vehicle. Disc brake systemsgenerally include a rotor and caliper. The rotor is mounted to turn withthe wheel of the vehicle. The caliper includes brake pads that areforced into frictional contact with the rotor to slow and or stoprotation of the wheel. Conventional cast iron brake rotors arerelatively heavy. These rotors wear during braking, generating dust.Alternatives to conventional cast iron brake rotors can reduce weight aswell as contribute to better fuel economy, reduced both air and waterpollution, and enhanced vehicle acceleration. Reduced weight materialsto cast iron include aluminum and titanium; however, their surfacetribology in a friction application, in contrast to cast iron, lacks thenecessary performance to function as a brake rotor. It has been known toadd ceramic particulate to a metal matrix to increase friction forimproved stopping power and to enhance wear resistance, which also hasthe advantage of producing little to no dust in a friction application.

Many conventional processes have drawbacks when applied to forming brakerotors with desirable features. For example, some rotors Ruined ofaluminum with a ceramic coating applied by a conventional plasma spraytechnique have unsatisfactory residual porosity as well as insufficientadhesion of the coating to the rotor substrate for vehicular rotorapplications. Brake rotors mostly free of porosity are enabling forreliability and good heat transfer.

Conventional stir casting used to produce composites and particularlyaluminum-ceramic particle composites inherently results in unacceptableporosity for most vehicular rotor applications. In addition, ceramicparticle loading of at least 30 to 50% by volume is desirable for highwear resistance and braking performance of vehicles with relatively highmomentum. Conventional stir casting is not known to produce pore freecomposites at this necessarily high particle loading. In conventionalstir casting, ceramic particles are incorporated throughout a moltenaluminum alloy. As the insoluble particulate is stirred into the liquidaluminum, the viscosity of the mixture increases with the volume ofparticulate added. The increase in viscosity is also related to the sizeof the particulate. Generally, the smaller the particulate, the greaterthe increase in viscosity. However, fine particle sizes are used,preferably below approximately 50 μm for good braking performance. Aconventional stir process to incorporate fine ceramic particles into thealuminum matrix for brake rotor applications is limited to approximately25% by volume, depending on the actual particle size, because stirringand casting of higher volume loading and viscosity mixtures becomeimpractical.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand apparatus for forming substantially porosity free metal/ceramiccomposite bonded on support substrates that overcomes the aforesaid andother problems with the prior art.

According to various aspects of the present invention, a composite layerformed on a surface of a substrate (e.g., a vehicular brake rotor),using the systems and methods discussed herein, is substantially free ofporosity and includes ceramic particles of desirable size and compositedensity. A relatively high density of ceramic particles within thecomposite layer has a capability to retard heat flow into the coresubstrate. The layer may encompass the entire substrate surface or beformed in one or more selected regions (e.g., one or more circularbands) of an external surface of the substrate. The composite layer mayalso provide reinforcement to the substrate, so that the combination ofthe layer and substrate provides an improved combination of mechanicaland thermal properties (e.g., friction coefficient, wear rate, thermalconductivity, thermal capacity, specific heat, specific gravity,density, compressive strength, ductility, stiffness) compared to thesubstrate without the layer.

In an application of the inventions discussed herein related tovehicular brake rotors, the layer depending on particle volume loadinghas higher friction and inherently has higher heat capacity than castiron. The higher heat capacity provides a greater heat storage capacitythan iron, a ceramic particle loading higher than approximately 30volume percent provides improved braking performance, and low wearcharacteristics results in little or no dust generation, and thus lessair pollution than cast iron.

The composite friction layer may be graded as to density or volume ofceramic particles as a function of distance from the outer surface ofthe layer toward the substrate. More than one layer may be formed. Forexample, when the ceramic particle volume is varied from the outersurface to the substrate core, the temperature of the substrate may beless than the temperature of an exterior surface. A thin coating on theexternal surface by conventional means that includes a relatively lowthermal conductivity ceramic (e.g., oxides) may provide excellentthermal insulation of heat flow to the substrate.

Substrates of aluminum, aluminum alloy, titanium, and/or titanium alloyare much lighter than cast iron and are preferred because of their lightweight and high heat capacity for vehicular brake rotor applications.Titanium, similar to aluminum, does not provide a tribological surfacesuitable for vehicular brake rotor applications. A layer of ceramicparticles in a metal matrix of one or more metals (e.g., metals of thesubstrate, or metals having lower melting points than the substratemelting temperature, and metals serving as reactants in an intendedexothermic reaction) may provide a wear resistant composite brakingsurface capable of withstanding vehicular braking temperature extremes.

In the description below, reference is made to vehicular brake rotors asan example of a metal substrate coated with a metal matrix compositelayer. The metal substrate is also called a core. In operation, anexternal surface of the layer provides a braking surface. When such abrake rotor is mounted on a wheeled vehicle, in a conventional fashion,as discussed above, friction between the braking surface and one or morebrake pads slows and/or stops the vehicle.

A system for forming a layer onto a surface of a solid metal substratemay be assembled from conventional manual and/or automated processingand process controlling equipment and facilities configured according tothe disclosures herein. Automation may be accomplished by installing inthe equipment, or linking the equipment for access to, firmware and/orsoftware prepared using conventional programming technologies. In oneimplementation, a system includes an integration of a chamber, a heater,a press, and a vacuum pump of conventional design. A system may be ofthe type known as a vacuum hot press. Commercial vacuum hot presses ofthe type supplied by Thermal Technology LLC of Santa Rosa, Calif. andThe Furnace Source LLC of Terryville, Conn. may be used, or custom madesystems can meet the objectives of producing metal matrix compositebrake rotors.

A chamber includes any enclosure insulated to economically maintain arelatively high temperature within and relatively low atmosphericpressure within, while located in ambient conditions without. Thefurnace operation may also be performed under positive pressure or witha gas flow, as long as an inert gas such as argon or a gas that issubstantially unreactive with the brake materials such as nitrogen isused. Generally, a chamber includes a closable opening through whichopening a die may be placed in the chamber to be subject to the highertemperature and applied pressure for a desirable period and laterremoved from the chamber. Additional passages through the enclosure aregenerally useful for supporting the heating, pressing, and evacuatingfunctions; as well as instrumentation and controls. A chamber mayinclude a structure to accommodate a press. Conversely, a chamber may bemoveable and lowered or alternatively raised over a base with ceramiccomposite layers to be processed into a brake rotor.

A heater includes any apparatus (e.g., electrical, chemical, magnetic)that provides energy to heat the contents of the die. Heating may beconducted into the die (e.g., pressing a hot object against the die,electrical conduction through the resistance of the die and/or itscontents, magnetic induction and heating of the type called spark plasmasintering also referred to as field assist sintering technology (FAST)or current activated pressure assisted densification) and/or radiatedtoward the die (e.g., microwave heating, electron beam heating, laserlight heating, radiant heat source within the chamber, and conduction orconvection via a gas that surrounds the die prior to evacuation of thechamber). The heater may include a source of energy used for heating.The heater may have access to a source of energy used for heating orenergy conversion. The heater may convert energy from one form toanother (e.g., electrical power to RF for magnetic induction, pulsing ofpower, etc.)

A press includes any equipment that applies a mechanical force onto adie within the chamber. A press may include one or more rams (e.g.,moving piston, fixed backstop) that come in contact with the die toapply the force. The press may include heating, cooling, and/orinsulation to reduce thermal variation between the press and the die.

A vacuum pump includes any mechanical equipment that removes gas from achamber. A vacuum pump may also replace the gas in the chamber withanother gas. A vacuum pump may govern a pressure of gas surrounding thedie.

Integrating control of the temperature, mechanical pressure, and extentof surrounding gas within the chamber may be accomplished by one or moreprogrammed controllers. Programming may include sequences for treatingthe contents of the die and sequences for maintaining safe operatingconditions for persons, equipment, and the contents of the die.Sequences, in any conventional programming language, may includeinstructions (e.g., commands, parameters, arguments, statements) thatdefine set points, rates of change, durations at set points, durationsfor accomplishing changes, limit conditions for exceptional actions whenthe limit is crossed (e.g., return chamber to ambient conditions, returnchamber to known safe conditions, sound alarm for operator to makeadjustment, end sequential control), and/or program sequence controls(e.g., branching, looping, subroutines, error handling). Programming maybe centralized or distributed. When program control is distributed amongspecialized controllers, signals conveying status and controls (e.g.,feedback, commands, requests for information, requests for resources,commitments) may be sent and received using conventional technologies toaccomplish the effect of centralized program control.

DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be seenfrom the following detailed description, wherein:

FIG. 1 is a functional block diagram of a system that can be used toproduce a dense, essentially porosity free, metal ceramic particulatecomposite in the architecture of a composite coating on a core metal oras a component that encompasses the entire cross-section of thecomposite according to one aspect of the present invention; FIG. 1includes a cross sectional view of a die and its contents, according tovarious aspects of the present invention

FIG. 2 is a flow chart of a method for forming a layer onto a surface ofa substrate, according to one aspect of the present invention;

FIG. 3 is top view of an empty die for use in the system of FIG. 1and/or with the method of FIG. 2; and

FIG. 4 is a top view of a press fit base for use with the die of FIG. 3;a press fit top having similar construction.

FIG. 5 is a perspective view of a press fit tip and bottom assembledtogether; and

FIG. 6 is a view similar to FIG. 2 of an alternative method for forminga layer onto a surface of a substrate according to another aspect of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 1 shows a system that can be used toproduce a dense essentially porosity free metal ceramic particulatecomposite coating on a metal core in accordance with the presentinvention. As used herein “porosity free” means that the coatingcomprises generally not more than about 0.05-2% by volume pores, moreparticularly about 0.5-1% by volume pores, even more particularly about0.1-0.25% volume pores.

System 100 of FIG. 1 includes chamber 102, controller 110, heatingsubsystem 112, pressing subsystem 114, and evacuating subsystem 116.These components of system 100 may be arranged in any convenientorientation to each other and to gravity. As shown, die 150 is locatedin chamber 102 for processing contents of die 150. Die 150 definesinterior 166. Die 150 includes peripheral component 152, press fit base156, and press fit top 154. Contents of die 150 are placed in dieinterior 166 and include solid substrate 162 and powder 164 and 165.Placement of the contents into interior 166 is referred to herein asloading die 150. Loading may be accomplished with any conventionaltechnologies manually and/or automatically.

Chamber 102 represents a chamber as discussed above. Chamber 102provides a thermally insulated interior 104. Chamber 102 isolates theinterior 104 from ambient atmosphere. Chamber 102 has a door and adoorway (not shown) through which a loaded die may be placed into thechamber for processing with the door closed. Placement may be by manualand/or automatic means of conventional design (e.g., conveyer, hoist,elevator, turntable, web). When processing is completed, the die may beremoved and unloaded to release a product comprising a substrate with acomposite layer as discussed above.

Controller 110 represents a programmable controller as discussed above.Portions of controller 110 may be integral to one or more of heatingsubsystem 112, pressing subsystem 114, and/or evacuating subsystem 116(e.g., efficient packaging of instrumentation and/or circuitry forperforming instructions, for reporting status, for feedback control, formonitoring limit conditions, for alarms, for safety of personnel andequipment, for cooperation with materials handling equipment).Controller 110 includes conventional memory (not shown) and aconventional sequential machine that reads the memory to perform asequence as discussed above. In an implementation, controller includes apersonal computer, Microsoft Windows operating system, and processcontrol software. The personal computer includes a user interface forreceiving program instructions to store one or more sequences in thememory; and further includes displays for monitoring progress of theprocess applied to contents of die 150, observing conditions asdescribed by instrumentation that may be located within heatingsubsystem 112, pressing subsystem 114, evacuating subsystem 116, chamber102, and/or die 150. Controller 110 communicates with heating subsystem112 via signals 122 for status and control using conventionaltechnologies. Controller 110 communicates with pressing subsystem 114via signals 124 for status and control using conventional technologies.Controller 110 communicates with evacuating subsystem 116 via signals126 for status and control using conventional technologies.

Heating subsystem 112 includes a heater as discussed above. Heatingsubsystem 112 responds to controller 110 to provide heat 132 to thecontents of die 150 and/or ram 134. Heat from subsystem 112 melts powder164 and 165 as discussed below. Heating subsystem 112 may furtherinclude conventional cooling technologies controlled by controller 110to cool the contents of die 150 at any suitable time in the sequence.

Pressing subsystem 114 includes a press as discussed above. Pressingsubsystem 114 responds to controller 110 to apply a mechanical force tothe contents of die 150 via ram 134 and press fit top 154. As shown,pressure from ram 134 is passed through standoff 170 to avoid mechanicalinterference from hub 158. The force from pressing subsystem 114densifies the contents of die 150 as discussed below. The mechanicalforce applied by pressing subsystem 114 is matched by a reaction force.The reaction force may be provided by an opposing ram (not shown). Forsimplicity, FIG. 1 indicates the reaction force provided by earth wound106.

Standoff 170, cover 154, and press fit base 156 are designed to evenlydistribute the mechanical force of the ram onto the contents of die 150.In an implementation according to various aspects of the presentinvention, a uniform thickness of the layer is achieved by assuring thatthe contents of die 150 are maintained throughout the process relativelyclose to level.

Evacuating subsystem 116 includes a vacuum pump as discussed above.Evacuation subsystem 116 is in fluid communication by coupling 136 todie interior 166 via vent 172 through hub 158, through standoff 170, andchamber interior 104. Evacuation subsystem 116 responds to controller110 to control the kinds of gases (if any) and gas pressure in dieinterior 150. Evacuation subsystem 116 may similarly control chamberinterior 104, for example, to indirectly control die interior 166 viavent 172. Evacuation subsystem 116 may introduce and/or pressurize oneor more gases in die interior 166 and/or chamber interior 104.

A die includes any structure that contains contents subject totemperature and pressure. At least some of the contents may be in liquidphase. For example, die 150 contains (e.g., encloses) substrate 162 andpowder 164 and 165 so that any liquid and/or gaseous phase of thesematerials that may exist during processing will not leak from die 150.For example, when powder 164 and 165 includes aluminum alloy, aluminumalloy in liquid phase (e.g., resulting from an elevated temperaturecaused by heating subsystem 112) is contained (e.g., enclosed) by die150 so as not to leak into chamber interior 104. Die 150 may be formedof several components with dimensions suitable for sealing at interfacesbetween its components.

Sealing prevents the low viscosity liquid metal phase of the contents ofdie 150 from leaking out of die 150. Seals described herein inhibitleaking of metals at the temperatures and pressures discussed below.According to various aspects of the present invention, the amount ofliquid phase contents of die 150 is relatively small compared toliquefying the entire content of die 150.

Die 150 may be formed of graphite, ceramic, and/or metal (e.g., steel,titanium) with a coating (e.g., carbon, graphite, boron nitride).

In one implementation, according to various aspects of the presentinvention, die 150 is formed of press fitted graphite components, sizedto be forced together in a friction fit of a graphite surface of a firstcomponent against a graphite surface of a second component.

Components may be friction fit into such materials to form suitableseals between components that prevent any liquid metal under pressurefrom leaking through such press fitted components.

Member 154 completes the enclosure of die 150 with respect to contentsin interior 166. Member 154 conducts mechanical pressure from ram 134via standoff 170 to contents of die 150. Member 154 also slides withindie interior 166 as urged by ram 134. In a first position, as shown,member 154 does not obstruct vent 172. In this position of member 154,vent 172 provides fluid communication between die interior 166 andchamber interior 104. Evacuation of chamber interior 104 consequentlyevacuates die interior 166 via vent 172. Introduction of a gas intochamber interior 104 also introduces the gas into die interior 166 viavent 172 by filling, mixing, and diffusion.

In one implementation, ram 134 applies a force only in one direction.Ram 134 may be used for three functions. First, ram 134 may be used tointroduce member 154 into die 150 to close die 150, applying a force toovercome the friction fit of member 154 and peripheral component 152.Member 154 is then in a first position. Second, ram 134 may be used tomove member 154 away from the first position, in the same direction(further into die 150) to obstruct vent 172 to prevent molten metal fromescaping through the vent as pressure is applied. Member 154 is then ina second position. Third, ram 134 may be used to provide a force in thesame direction for densification of the contents of die 150.

When ram 134 urges member 154 away from the first position (as shown)and further into die 150, member 154, in a second position, completelyobstructs vent 172, providing a gas and liquid seal so that contents ofdie 150 do not leak into chamber interior 104. In one implementation,according to various aspects of the present invention, member 154, acomponent of die 150, is formed of graphite. Dimensions of member 154provide a friction fit inside peripheral component 152 as discussedabove.

A substrate or core provides structural rigidity to the intendedproduct. The substrate has a melting temperature that is higher than amelting temperature of at least one of the components forming thecomposite surface layer so that by applying an elevated temperature thatis less than the substrate melting temperature, to the contents of thedie, the surface layer material will form a melt while the bulk of thesubstrate will remain solid, i.e., not melt. There will be melting onthe substrate surface in contact with the composite layer that resultsin excellent bonding. A substrate may be formed of structural metal(e.g., aluminum, cobalt, copper, iron, nickel, titanium, vanadium, zinc)in commercially pure form or in some cases, an alloy. Alloys may includeany structural metal(s) and/or alloying material(s) (e.g., chromium,magnesium, manganese, niobium, silicon). The substrate can be commercialplate material precut to a brake rotor which exhibits inherently lowporosity or substrate 162 may be a cast or rolled disc of commerciallypure aluminum or aluminum alloy formed with low porosity.

A surface layer material which typically is in the form of a powder mayinclude at least one reactant for an exothermic reaction and may furtherinclude material to perform a function and/or enhance performance of afunction of the product. The function may rely on structural properties.The powder and/or surface layer may be referred to as a reinforcement.The product of substrate and surface layer may be referred to as a corewith surface reinforcement. A powder, which may also take the form or ametal ingot may include particles of one or more materials, eachmaterial being a component material of the powder. A powder when heatedmay participate in an exothermic reaction. Preferably, the exothermicreaction does not produce products in a gas phase. For example, powder164 and 165 are identical in composition. Powder 164 provides a surfacethat is integral with substrate 162 as a result of the methods discussedherein. Powder 164 and 165 includes a ceramic particulate (e.g., siliconcarbide) to improve the friction and wear properties of a vehicularbrake rotor formed with substrate 162 as a core. Powder 164 or 165 mayinclude two reactants. The first reactant may be a structural metaland/or an alloying material. The second reactant may be boron, silicon,cobalt, iron, nickel, magnesium, palladium, titanium, or a combinationthereof. The exothermic reaction will not begin until the powder isheated to an elevated temperature, determined by the intended reactionand the reactants using conventional analysis, simulation, or test.

For vehicular brake rotors, silicon carbide is a preferred ceramicparticle included in the metal powder. Other ceramic particles that maybe used, in place of silicon carbide or in addition to silicon carbide,in the powder include metal carbides, nitrides, silicides, oxides, andintermetallic compositions. The metal component of these ceramicparticles include aluminum, titanium, boron iron, nickel, cobalt,copper, and zinc solely or in any combination. In an implementation forthe fabrication of vehicular brake rotors, the powder may include analuminum alloy and a ceramic particulate. The aluminum alloy may includeSi from zero percent up to 6% to 19% by weight, preferably 7 to 18% byweight, more preferably 9 to 12% by weight and Mg from zero up to 0.1%to 15% by weight, preferably 0.2 to 12% by weight, more preferably 0.4to 1% by weight. The silicon of the alloy provides excellent wettabilityand/or bonding to most ceramic particles including silicon carbide.Silicon coupled with SiC reduces the formation of aluminum carbide.Aluminum carbide can corrode in the presence of moisture, thereby losingstructural integrity. In a preferred vehicular brake rotor, according tovarious aspects of the present invention, the composition of the layeris 25% to 65% by volume or more preferably 30 to 65% by volume, and morepreferably 35 to 50% by volume of single or mixed ceramic particulate.

A layer refers to a result of the bonding of the metal matrix and thesubstrate. A cross section of a product of a substrate and a layerformed according to various aspects of the present invention may reveala graded concentration of a component material of the powder beginningat the surface of the product and extending into the substrate. Thelayer corresponds to the portion of the product having more than orequal to a concentration of the component material that is significantto serving a function of the product. The interior boundary of the layermay be difficult to ascertain by visual inspection. However, based onthe intended function, concentration of component material needed tosignificantly perform that function, and electron dispersivespectrometry, a boundary (e.g., depth from the surface) may bedetermined. Because the boundary may not be apparent to visualinspection, the boundary may be determined by analysis, simulation,and/or test and specified as a dimension (e.g., thickness) of theproduct. For example, for a vehicular brake rotor as the product, alayer formed by powder 164 may extend into the product from the surfaceof the product to a boundary defined with reference to a concentrationof ceramic particles associated with an end of useful braking life ofthe vehicular brake rotor. A concentration of silicon carbide from thepowder may be determined by test at various depths of a prototypeproduct so as to define the braking layer. A method for forming a layeronto a surface of a solid substrate, according to various aspects of thepresent invention, produces the layer substantially absent of porosityas discussed above. This result is obtained, in some implementations, bya combination of one or more of evacuating the die cavity, heating thecontents of the die to a temperature between the melting point of themetal powder and the melting point of the substrate, and applyingpressure for a period that may begin before any potential exothermicreaction between the powder and the substrate and may end after thelayer has cooled below the melting point of the powder. In animplementation of the method to produce a vehicular brake rotor, thelayer comprises a ceramic joined to the substrate by integration withthe substrate to faun a metal matrix composite. The layer may havesuperior friction, wear, and thermal capacity characteristics comparedto the substrate.

An implementation of such a method includes in any practical order,loading contents into a die where the contents include a solid substrateand a powder; heating the contents to a temperature that enables meltingof the surface of the substrate and the powder; and densifying thecontents. In another implementation heating in the absence of anexothermic reaction is performed that achieves bonding between thesurface layer and the substrate. The method may further includeevacuating the die before densifying.

For example, method 200 of FIG. 2, begins with assembling (202) a die(150), coating the interior of the die with a mold release compound, andplacing the assembled die within the chamber. Assembly may be omitted(e.g., unnecessary) when a die of unitary construction is used, forinstance a die capable of containing the contents (in any phaseoccurring during the process) and product, as discussed above.

Depending on the materials used for the die, the powder, and thesubstrate, a suitable release compound may be applied to coat theinterior of the die. Such a coating facilitates removal of the productfrom the die. For example, a dry film lubricant formed of graphite andthermoplastic resin of the type used as a billet lubricant in hotforging of aluminum, orbital forging, and cold extrusion as well as moldpre-treatment in metal casting may be used. For instance, DAG 386marketed by Acheson may be used. Boron nitride release compounds alsomay be used.

Loading the die with contents may be accomplished by spreading (204) apowder (164) inside the die (150), placing (206) a substrate (162)(e.g., a brake rotor core) onto the powder (164), and spreading (208)powder (165) to cover the substrate (162). Aluminum ingot in solid formor in powdered form such as A356 or A359 may also be used in place ofthe mixed metal powders. The two powder distributions or two ingots orboth powders and ingots and the substrate constitute the contents of thedie.

In another implementation, one of the spreading steps may be omitted forforming a layer on only one side of the substrate.

Prior to loading the die, the powder may be prepared as a uniform mix oftwo or more powdered materials. Uniformity of particle size distributionas well as uniformity of particle material distribution within the mixmay be accomplished by preparing the powder at least in part usingconventional pulverizing, screening, and mixing technologies.

After loading, the die (150) may be covered (154) as discussed above. Inone implementation, the die (150) is closed leaving a vent (172) of thedie open for evacuation of residual atmospheric gases from the interiorof the die. The die may be placed in a vacuum hot press, a system (100)as discussed above, or a vacuum chamber for evacuation.

In the case where the die includes a vent and the vent is open, gases inthe interior of the die may be removed by evacuation (210) and/orpurging in one or more cycles to obtain a suitable absence ofatmospheric or other gas that could otherwise contribute to porosity inthe layer. Any introduced gas may remain to participate in a desiredchemical reaction or, preferably, be evacuated from the die. Afterevacuation, the vent is closed, for example, by movement of the ram asdiscussed above. In other implementations, the vent is closed bycovering the vent using conventional techniques (e.g., introducing aplug into the vent, applying a band around the die to secure the plug,applying a band around the die to cover the vent).

In one implementation, evacuation (210) is accomplished in a vacuumchamber (as opposed to a vacuum hot press) equipped to close the vent.In another implementation, the die is loaded (204, 206, 208) and closedin an evacuated chamber. In still another implementation, the die doesnot include a vent and the die is loaded (204, 206, 208) and closed(e.g., cover 154 installed) in an evacuated chamber. The die is thenplaced in a conventional hot press or a system of the type discussedabove with reference to FIG. 1 except that the evacuation subsystem isomitted with consequent simplifications to the controller and chamber).

One or more loaded dies (150) may be prepared in advance, transported,and/or stored indefinitely. Loaded dies may be combined into a web forbatch processing. Batch processing may be accomplished with conventionalmaterials handling technologies between any steps of method 200. Thecontents of the loaded die are heated (212) to an elevated temperaturesufficient to melt the metal alloy in the surface layers. Any exothermicreaction between component materials of the powder will further increasetemperature in the surface layer so that less external heating isrequired, producing what is herein called a melt. Generating a highertemperature at the interface between the surface layers and the corewill also contribute to better adhesion between the surface layers andthe core. However, for certain component materials such as Al—SiCpowder, the exothermic reaction will either be very small or not occurat all unless e.g., Ti is added. The heating may be continued until theentire die contents are at the elevated temperature. Heating may bediscontinued after a period sufficient to fully melt the surface layersand bond to the core and for substantially completing the exothermicreaction of the contents of the die (e.g., time required for 90% to 100%of the intended reactants to be consumed by the reaction). Theexothermic reaction may also involve a surface of the substrate (162)adjacent to the powder. Enablement generally includes exceeding themelting point of a component material of the powder.

In the absence of the substrate, the powder has a melting temperature atwhich at least one component material of the powder exists in a liquidphase. The powder melting temperature may be defined as a temperature atwhich one or more component materials of the powder exist in a liquidphase. When more than one component is melted, the melted components mayform an alloy or a new compound. For example, molten aluminum alloy mayreact with titanium powder to form one or all the intermetallics TiAl,Ti₃Al, TiAl₃. Remaining component materials of the powder (generally,ceramic particulate) may continue in a solid phase. Exothermic reactionsmay improve the wetting of the solid materials of the powder with theliquid phase materials of the powder.

The surface of the substrate when not in contact with the powder has amelting temperature, herein called the substrate melting temperature.The surface of the substrate may be subject to the heating and chemicaleffects of the exothermic reaction and experience localized melting atits surface or that in contact with the lower melting temperaturesurface layer. For example, silicon in aluminum alloys lowers itsmelting temperature. If Si is present in the surface layer with Al, theSi could begin to react with the surface of the core and diffuse intothe core to cause further melting. Thus, the time at the meltingtemperatures during processing is important and must be optimize toavoid inching the core beyond the surface of the core. Extended times atthe melting temperature could also cause porosity at the interface,which is detrimental to mechanical and thermal properties.

In an important class of implementations, according to various aspectsof the present invention, the bulk of the substrate does not exist as aliquid at any time during the process. To avoid bulk melting, theelevated temperature discussed above, must be below the substratemelting temperature. To avoid bulk melting, the elevated temperature,duration of maintaining the elevated temperature, extent of anyexothermic reaction, and the thermal capacity of the substrate arcselected by analysis, simulation, and/or test.

Before, during, and/or after enabling any potential exothermic reaction,the contents of the die may be densified. Densification may beaccomplished using techniques somewhat similar to hot press techniques.Hot press techniques apply pressure to solid materials. In contrast,pressure, according to various aspects of the present invention, isapplied to a melt that includes liquid. For example, pressure may beapplied (214) to the contents before and/or during any exothermicreaction that may occur. The pressure (i.e., a mechanical force) maycontribute to initiation and/or maintenance of the exothermic reaction.The pressure may be applied to a component of the die (e.g., cover 154,base 156, both cover and base). The pressure may be applied to force thecover and base of the die toward each other. Pressure may continue atthe same force or at a series of forces. Pressure may be interruptedbriefly and reinstated, for example, to facilitate movement of the dieout of a region for heating (104) and into a region for cooling.

By applying pressure to the melt against the substrate possibly duringthe exothermic reaction, at least one material of the melt (e.g., ametal matrix composite, an intermetallic composite, a liquid metalalloy, a product of the reaction) bonds with the substrate by forming ametal matrix composite, an intermetallic composite, and/or an alloy ofthe substrate. By applying pressure to the melt, solid particulatematerial of the melt may be more thoroughly wetted. Consequently, whenthe product is in use, the layer may retain the solid particulate inadverse conditions.

The exothermic reaction, presence of the melt against the surface of thesubstrate, and/or pressure of the melt against the substrate mayintegrate the melt and the surface of the substrate. Integrating mayinclude local melting of the surface of the substrate.

Preferably, densification continues as the contents of the die cool froma melted liquid component to a solid. For some substrate and powdercombinations, significant shrinking of the product may occur duringcooling from a liquid to a solid. Porosity may be avoided by continuingdensification pressure during cooling. For example, pressure ismaintained on the contents of the die while the contents of the die arecooled (216). Accelerated cooling may be accomplished using conventionaltechnologies (e.g., introducing chilled gas into the chamber such asfrom a supply of liquid nitrogen or merely a gas flow through thechamber).

After a risk of further significant shrinkage is low, pressure may beremoved (218). Thereafter, the chamber (102) may be prepared foropening. The die may be removed from the chamber to facilitate removalof the product from the die. Finally, the product is removed (220) fromthe die. Removal may include disassembly of the die. Removal may includeremoving a peripheral component from the remaining combination of thedie and the product, then separating the base and cover components fromthe product. The components of the die may be refurbished for reuse orreused as-is.

In one implementation, according to various aspects of the presentinvention, the product is ready for installation in a next assembly orready for use. To meet product specifications, post-process machiningmay be unnecessary. In other implementations, the product is in near-netshape, ready for post-process machining.

In an implementation for forming a vehicular brake rotor, a die servesas a containment vessel for molten metal. All components of the die maybe formed of similar constitution to avoid differences in thermalexpansion. For example, die 150 of FIG. 3 includes peripheral component152, bottom base component 156, hub component 158, and top covercomponent 154 of FIG. 4. In one implementation, the components have thedimensions and characteristics described in Table 1 below.

A peripheral component provides one or more exterior side walls for adie. The exterior side walls may correspond to a net shape desired forthe product. Circular and oval shapes are preferred to simplify removalof the product from the die and for better containment of molten metals.For example, peripheral component 152 is formed as an empty cylinderhaving an axis of circular symmetry through center 302. Peripheralcomponent 152 defines at radius 386 an exterior surface of die 150.

A base component serves to at least partially close a peripheralcomponent so that the die may be loaded. A base component may provide asurface for the application of mechanical force against the contents ofthe die. For example, base component 156 is formed as a disc having anaxis of circular symmetry through center 302 that corresponds to theaxis of symmetry of peripheral component 152. Base component 156 forms acircular friction fit seal at radius 384 with peripheral component 152.Base component 156 includes a circular orifice sized to form a frictionfit seal against hub component 158 at radius 382. The friction fit sealbetween components is adjusted such that at operating temperatureexpansion does not result in relaxing the friction fit that allowsliquid metal under pressure to leak through the friction fit.

A hub component provides one or more interior side walls for a die. Theinterior side walls may correspond to a net shape desired for theproduct. Circular and oval shapes are preferred to simplify removal ofthe product from the die. For example, hub component 158 is formed as acylinder having an axis of circular symmetry through center 302. Hubcomponent 158 forms a circular friction fit seal at radius 382 with basecomponent 156. Hub component 158 includes a vent 172.

Vent 172 may be formed by drilling one or more holes into hub 158. Inother implementations, hub component 158 includes more than one vent ofsimilar construction and orientation.

A cover component for a die cooperates with one or more other componentsof the die to fully contain the contents within the die. A cover mayprovide a surface for the application of mechanical force against thecontents of the die. For example, cover component 154 is formed as adisc having an axis of circular symmetry through center 401 thatcorresponds to the axis of symmetry of peripheral component 152. Covercomponent 154 forms a circular friction fit seal at radius 384 withperipheral component 152. Cover component 154 includes a circularorifice 402 sized to form a friction fit seal against hub component 158at radius 404 and 382.

TABLE 1 Component Dimensions (inches) Peripheral component 152 Wallthickness 3.0 Height 3.0 Outside diameter 18.0 Base component 156Thickness 1.0 Diameter 12.0 Orifice diameter 6.0 Hub component 158Diameter 6.0 Height 3.0 Cover component 154 Thickness 1.0 Diameter 12.0Orifice diameter 6.0 Radius 382 of hub component 158 1.5 Radius 384 ofbase component 156 6.0 Radius 386 of peripheral component 152 9.0 Radius404 of orifice in cover component 3.0 154 Radius 406 of cover component154 6.0 Vent 172 diameter 0.0625 inch to 0.25 inch

Friction fit seals may be formed by mismatching the radii of thecomponents to be friction fit. The mismatch may be from 0.002 inch to0.010 inch, preferably about 0.003 to 0.009 inch, more preferably 0.004to 0.008 inch. A mismatch of about 0.005 inch is particularly preferred.The mismatch remains constant at elevated temperatures with section ofthe same composition and cross-section.

Further features of the invention will be seen from the followingworking examples.

EXAMPLE I

As a first example, a vehicular brake rotor may be formed using method200 as follows. At steps 204 and 208 the powder comprises powderedaluminum silicon alloy, silicon carbide powder, and titanium powder.There is no binder in the powder. The AlSi alloy component material is59.5% by weight of the type marketed by Valimet as Grade S-9 having adistribution of particle sizes defined by D10 4.82 microns, D50 15.23microns, and D90 37.56 microns. The SiC component material is 38.0% byweight of the type marketed by ESK as Grade F600-D having particle sizefrom 10 microns to 20 microns. The titanium component material is 2.5%by weight of the type marketed by ADMA having particle size of about 44microns specified as 325-mesh. The powder is mixed until uniform inmaterial distribution. Each powder portion 164 and 165 is spread to auniform thickness of 0.010 inch to 0.25 inch, preferably 0.060 to 0.125inch, more preferably 0.040 to 0.080 inch, most preferably about 0.050inch.

The titanium component material of the powder serves several purposes.Titanium and aluminum react when the aluminum is in liquid phase,producing heat (i.e., an exothermic reaction). The reaction of titaniumand aluminum may form titanium aluminide having a melting point muchhigher than the elevated temperature of the chamber. Consequently, someof the aluminum of the powder, now as titanium aluminide, is no longerin liquid phase, reducing the risk of leaks through the seals of die150. The surface of the layer is subject to the formation of pipingstructures as the melt cools, a result of surface deformation due toshrinking. The formation of piping structures may be inhibited byincluding the titanium component material in the powder and applyingpressure during the cooling of the aluminum alloy component.

At step 206 the substrate is commercially pure aluminum (wroughtaluminum) also refeiTed to as 1100 aluminum in the shape of a disc about12.0 inches in diameter and about 0.5 inch thick. The disc has a centralcircular aperture about 6.0 inches in diameter for mounting the disconto a conventional hub of an automobile. The commercial aluminum dischas very low porosity.

At step 210 the die is evacuated using a conventional pump down scenarioto a pressure less than 100 torr. The chamber is back filled withnitrogen or argon gas and then pumped down a second time. The chamber isfilled with nitrogen or argon gas and then pumped down a third time. Theresulting pressure is less than 100 torr. Prior to evacuation, thepowder in the die may be dried. To accomplish drying, the heatingsubsystem 112 is programmed with a set point of between 150 degrees and250 degrees Celsius, preferably about 200 degrees Celsius and a holdtime of up to 10 hours to 20 hours. The vent is closed by programmingthe pressing subsystem to a set point of from 20 psi to 100 psi tosmoothly move the cover from the first position to the second positionas discussed above.

In another implementation, the powder is pre-dried by any conventionaltechnique (e.g., vacuum, temperature) before being spread into the die(204, 208). Consequently, step 210 may be simplified by omitting thetemperature set point and hold time.

At step 212 the heating system is programmed with a set point of about625 degrees Celsius and a hold time of about up to 1.5 hours. A setpoint of 550 degrees Celsius and a hold time of about 2.0 hours may alsobe used. A set point of 750 degrees Celsius may be used for a periodthat ends when a temperature of hub component 158 reaches a temperaturefrom 550 degrees to 625 degrees Celsius; followed at the end of theperiod by a set point of 625 degrees Celsius and a duration of from 1.5hours to 2.0 hours. As discussed above, the substrate comprises a metalin pure or alloy form having a substrate melting temperature that ishigher than a temperature that melts a component material of the powder.The contents of the die are heated in a manner that does not result inbulk melting of the substrate.

Steps 212 and 214 are performed concurrently.

At step 214 pressing subsystem 114 is programmed to a set point of from250 psi to 600 psi, preferably from 350 psi to 600 psi more preferably500 psi and a rate to achieve a transition time of from 0.25 to 1.0hour, preferably about 0.5 to 1.0 hour, more preferably about 0.5 hour.The linear displacement of ram 134 within die 150 is limited to thecalculated thickness of the contents when in liquid form. After coolingbegins in step 216, an additional linear displacement into die 150 maybe calculated to account for shrinking of the product during cooling.

At step 216 the heating system is programmed with a set point of ambienttemperature. Cooling proceeds for about 2.5-3 hours without forcedcooling at which time the die is about 300 degrees Celsius. Preferably,densification continues through cooling in the evacuated chamber.

In another implementation, after a final soak at 625 degrees Celsius,the evacuating subsystem is programmed with a set point of ambientpressure and further cooling is accelerated by introducing nitrogen intothe chamber from a liquid nitrogen source.

At step 218, the pressing subsystem is programmed with a set point ofzero psi or the pressure is simply released manually. When the set pointis achieved, the die is removed from the chamber.

At step 220 die 150 is disassembled to remove the formed disc, which maythen be welded to a rotor hub for installation on a vehicle. Theresulting vehicular brake rotor product has a solid aluminum core about8 mm thick and a layer about 3 mm thick on each side comprising siliconcarbide in an aluminum matrix for the braking surface. From 35% to 60%of the braking surface area on each side of the rotor is a siliconcarbide metal matrix composite. The porosity of the rotor is preferablyless than 0.5-1% by volume. The bond formed between the metal matrixlayer and the core is a metallurgical diffusion bond which eliminatesdelamination and spalling. As a result of the method, evacuationdegassed the melt, applied mechanical force maintained the solid siliconcarbide particles in the melt, and applied mechanical force contributedto bonding of the melt to the surface of the substrate.

Vehicular brake rotors, formed using the systems and methods discussedherein, may include a ceramic composite through the full thickness ofthe rotor that may include an exothermic component of the formulation.In other implementations no exothermic component is utilized in theformulations and the ceramic composite portion of the rotor exhibits agradation of the ceramic particle from a relatively high concentrationon the friction surface to a relatively lower concentration through thethickness of the rotor.

EXAMPLE II

As a second example, a vehicular brake rotor may be formed in avariation of method 200 as follows. At steps 204 and 208, the powdercomprises aluminum alloy, titanium, and silicon carbide. The aluminumalloy is of the type generally referred to as 356. Silicon carbide isincluded at 35% of the powder. Ninety percent of the silicon carbideparticles are 15 micrometers (μm). The particles are dry mixed by anyconventional mixing technology (e.g., ball mill with steel balls orceramic balls, attrition milling, rotational tumbling, pulse fluid bed).After mixing to a uniform material consistency, the powder is pouredinto a die to contain the mixture when heated to above the melting pointof the aluminum alloy of the powder.

At step 206, a plate of pure aluminum produced by casting or rolling isused as the substrate and aluminum alloy 356 used as the surface layers.

At step 210, the loaded die is placed in a chamber capable of evacuationof air. A vacuum pump is used to pump down the pressure in the chamberto under 100 Torr. The chamber is back filled with an inert gas (e.g.,nitrogen, argon, carbon dioxide, ammonia). The pump down and backfilling cycle is repeated two additional times.

Steps 212 and 214 are performed concurrently.

At step 212 the temperature is raised to the melting point of aluminumalloy 356, thereby melting the aluminum alloy of the powder and meltingthe surface. The titanium powder reacts with the molten aluminum of thepowder and the substrate exothermically (a thermite type reaction)producing heat that contributes to the melting of the aluminum alloy ofthe powder. The heat also enhances the wetting and/or bonding of thealuminum with silicon carbide particles and the wetting and/or bondingof silicon with the silicon carbide particles. The titanium componentmaterial of the powder reacts with the molten aluminum alloy of thepowder to form titanium aluminides (e.g., TiAl, Ti3Al and TiAl3). Someof the titanium component material of the powder may react with siliconfrom the aluminum alloy of the powder to form titanium silicides (e.g.,TiSi2, Ti5Si3).

At step 214, pressure is applied to the die with a leak proof fitted ramthat prevents any squeeze out of liquid aluminum metal. Pressures fromapproximately 1.4 MPa to 35 MPa (200 psi-5 ksi) may be used. Theselected pressure is sufficient to cause the liquid aluminum alloy ofthe powder to infiltrate the remaining component material of the powder.The pressure of the ram on the molten aluminum alloy presses onto thesilicon carbide particles. In a first implementation, sufficientpressure and duration (e.g., suitable set points) concentrate thesilicon carbide particles at the surface adjacent to base 156 to providerelatively higher friction on that surface as compared to the surfaceadjacent to cover 154. In a second implementation, a lower pressure setpoint and/or shorter duration set point is used compared to the firstimplementation. In the second implementation, silicon carbide particlesare more uniform throughout the cross-section of the product.

At step 216, heating is discontinued. Cooling proceeds to ambienttemperature.

At step 220 the product, a vehicular brake rotor, is removed from thedie. The product preferably exhibits substantially no porosity. Nosubsequent machining is required for use of the product in a vehicularbrake system.

The product consists of a composite formed in part by an exothermicreaction of titanium particles and molten aluminum alloy, where thecomposite is a result of heat from the reaction, auxiliary heating, andpressure pushing ceramic particulate through molten aluminum.

In a first implementation, steps 210 continues concurrently with steps212, 214 and 216. In a second implementation steps 212, 214, and 216 areperformed in ambient atmospheric pressure. Although both productscontain substantially low porosity, lower porosity can result from thefirst implementations, compared with the second.

Several variations of this second example are implementations accordingto various aspects of the present invention. Any aluminum alloy may beused for the powder and the substrate as long as the melting temperatureof the substrate is higher than the aluminum alloy used for forming thelayers. The amount of SiC powder spread in the die at steps 204 and 208may be increased to increase the surface concentration or loading ofceramic in the product. Pressing may be performed under reducedatmospheric pressure, or at ambient atmospheric pressure of an inertnon-reactive gas. Heating may be accomplished using conventionalelectrical conduction, magnetic induction, radiant microwavetechnologies or resistant heating of the die contents through an upperand lower rain. The powder may include reactants in place of or inaddition to titanium to support exothermic reactions with aluminuminvolving cobalt, iron, magnesium, and/or nickel. The powder may includeceramic particles of size suitable for the product function. Forvehicular brake rotors, ceramic particles may be used having size lessthan 500 microns, preferably less than 100 microns and most preferably50 microns and below. Any ceramic particle may be used in place of or inaddition to silicon carbide. In other implementations, ceramic particlesare a mix of different conventional materials (e.g., oxides, carbon,titanium carbide, chromium nitride, chromium carbide, titaniumdiboride).

EXAMPLE III

As a third example, the process of Example II was repeated, except thesubstrate was formed of aluminum alloy 5052, and the metal alloy powderused in the surface layers was type 4032.

EXAMPLE IV

As a fourth example, a vehicular brake rotor may be formed by a method200 as follows. At steps 204 and 208, a mixture of aluminum powder andsilicon carbide powder is used.

At step 206, the substrate is a disc of commercially pure titanium,pre-cut to dimensions for a vehicular brake rotor.

Steps 212 and 214 are performed concurrently.

At step 212, resistance heating includes connecting an electric currentsource (e.g., DC, AC, pulse) supply to the ram and the base component ofthe die. Electrical current through the die raises the temperature,melts the aluminum powder, and generates the plasma. Heat causes thealuminum to melt. The bulk substrate does not melt because thetemperature experienced by the substrate does not exceed the meltingtemperature of the substrate. However, the surfaces of the substratewill start to react with the elements in the metal matrix compositesurface layers, which is beneficial as it promotes strong bonding at theinterface. The melted aluminum exothermically reacts with the titaniumof the substrate to form gamma titanium (i.e., TiAl) or other aluminidesof Ti₃Al or TiAl₃. During the liquid phase of the aluminum, a fullydense surface layer forms, consisting of silicon carbide particles intitanium aluminide (e.g., TiAl, Ti3Al, TiAl3).

At step 214, pressure is applied in the range of approximately 10 MPa to35 MPa (1.45 ksi-5 ksi) to densify the contents of the die. Withpressure applied, the silicon carbide particulate reinforced titaniumaluminide is made nearly fully dense, substantially free of porosity.

Several variations of this third example are implementations accordingto various aspects of the present invention. The powder may furtherinclude other alloying elements known to produce good gamma alloys(e.g., chromium, manganese, niobium, silicon). The feed powder may beprealloyed powders such as TiAl, Ti—Al—Cr—Nb, etc. in contrast to theinitial feed to Ti and Al powders with the SiC or other ceramicparticles such as TiC, TiB₂, B₄C, etc. The substrate may be formed ofanother alloy of titanium instead of the commercially pure titanium suchas alloy Ti-6Al-4VHeating can be supplemented by radiation (e.g.,microwave), magnetic induction, or other technologies as discussedabove. The resistance heating technology between the rams may bereplaced with one or more of the heating technologies discussed above.Any ceramic particle or combination of ceramic particles may be used inthe powder. The powder mixture may be free of exothermic reactioncomponents or may include other reactants for exothermic reaction(s)such as intermetallic formers (e.g., copper, cobalt, iron, nickel).

EXAMPLE V

As a fifth example, a vehicular brake rotor may be formed by method 200as follows. Steps 204 through 220 are the same as for the third example,discussed above with a few exceptions. At step 210 the die is notevacuated. Instead the die and the atmosphere of the chamber are filledwith an ionizing gas (e.g., argon, helium, nitrogen).

At step 212 resistive heating is applied for a duration until (andincluding) the time the aluminum material component of the powderreaches its melting temperature. This duration may be determined byanalysis, simulation, and/or test to account for the mass and heattransfer characteristics of the die and contents of the die, and anyeffects of the other system components (e.g., the ram). Concurrent withthe resistant heating melting the aluminum component a plasma isgenerated through the melt due to the voltage which may be pulsed asrequired to melt the aluminum. The plasma relatively quickly raises thetemperature of the disc and the melt, facilitating densification. Theplasma cleans the surfaces of the particles of the aluminum beforemelting and the un-melted, silicon carbide ceramic, facilitatingconsolidation and densification.

The plasma may be generated by conducting a high energy discharge ofelectricity from additional power supplies and/or a capacitor bankthrough the ram that ionizes the gas in the die.

In one implementation, resistive heat is omitted from step 212 andmelting is accomplished solely by heat associated with the plasma.

Several variations of this fourth example are implementations accordingto various aspects of the present invention. The substrate may be anystructural metal or alloy of structural metal as discussed above. Anyceramic particle or combination of ceramic particles may be used in thepowder. The powder may include other reactants for exothermicreaction(s) such as intermetallic formers (e.g., copper, cobalt, iron,nickel). The pressure of the ionizing gas within the die may be belowambient, ambient, or greater than ambient pressure.

EXAMPLE VI

A mixture of aluminum alloy 356 in powder form was mixed with powders oftitanium that was 2% of the sum of the aluminum and 35% SiC with thelatter in a particle size with 90% at 15 micrometers (.tm). Theparticles were dry mixed by suitable mixing including ball mill withsteel balls or ceramic balls, attrition milling, rotational tumbling,pulse fluid bed and poured into a die to contain the mixture when heatedto above the melting point of the aluminum. On top of the dry mix asolid plate of 356 aluminum alloy was placed. The loaded die was placedin a chamber that was capable of evacuation of air. A vacuum pump wasutilized to reduce the pressure in the chamber to under 100 Torr. Thechamber was back filled with an inert gas which included nitrogen,argon, CO₂ and ammonia, and repumped down three times. Pressure wasapplied to the die with a fitted ram and the temperature raised to themelting point of the 356 aluminum alloy. Pressures from approximately 10MPa to 35 MPa (1.45 ksi-5 ksi) are satisfactory to infiltrate the molten356 alloy through the dry powder mixture wherein the titanium powderreacts with the molten aluminum exothermically (a thermite typereaction) producing internal heat that contributes to the melting of the356 alloy powder and enhancing the wetting/bonding of the aluminum andsilicon with the SiC particles as well as the titanium reacting with themolten alloy that forms titanium aluminides that can include TiAl, Ti₃Aland TiAl₃, and the silicides TiSi₂ and Ti₅Si₃ from the Si in the 356alloy. The pressure of the ram on the molten 356 alloy presses onto theSiC particles that if high enough pressure and held long enough willconcentrate the SiC particles on the bottom surface that cansubsequently provide higher friction on that surface. If this pressureis in the lower range and the pressure removed after sufficient time forthe molten top 356 layer to infiltrate the SiC particle bed, the SiCwill be uniform through the cross-section of the formed composite. Heatpower is turned off and the molten 356 alloy cooled to room temperatureand the rotor removed from the die. Typically the surface of the meltmolded rotor is sufficient and the molding to tolerance that nosubsequent machining is required for mounting the rotor in the brakesystem.

The exothermic heating between the titanium particles and the moltenaluminum coupled with the auxiliary heating and the ram pressure pushingthe molten aluminum that is melted on top through the composite resultsin a composite with a residual porosity less than 2% and most often lessthan 1%, and typically less than 0.5% which is an ideal composite forbrake rotors.

One iteration that enhances the probability of low porosity is toperform the pressing under reduced pressure. Removing any residual gasby reducing the pressure contributes to no residual gas pockets beingtrapped leading to residual porosity.

Any aluminum alloy can be used as the matrix, any volume loading of SiCor any ceramic particle composition can be used, and the pressingoperation can be performed under reduced pressure or at ambient pressureof an inert non-reactive gas. The heating method can be by conduction,induction, microwave or any suitable means to raise the mixture in thedie containment to above the melting point of the aluminum. Also,reactions other than the titanium and aluminum can be used to generatean exothermic heat generating reaction. Some example exothermicreactions with the molten aluminum include Fe, Ni, Co, and Mg. Theparticle size of the ceramic filler can be any size, but smaller sizeparticles perform best in braking operations, therefore particle sizesunder approximately 500 μm should be used and preferably under 100 μmand most preferably under 50 μm. Also, mixtures of ceramic particles canbe used to achieve custom properties as the friction surface of thecomposite rotor. For example, oxide particles aid in resisting heat flowinto the body of the rotor in emergency braking scenarios, carbon canalso react with the titanium to form a TiC particle, chromium nitride orcarbide are known reducers for friction while TiB₂ increases thermalconductivity. It is therefore possible to customize the alloy andparticle compositions to produce brake rotors with customized propertiesthat includes friction coefficient and thermal conductivity of thesurface.

EXAMPLE VII

Referring to FIG. 6, a disc of CP titanium precut to brake rotordimensions was produced, and a mixture of aluminum and SiC powder aspreviously described was applied to both sides at step 602. Additionalmixture was placed in the bottom of a die, the CP Ti disc placed in thedie, and a layer of the mixture was added to the top surface of the CPTi disc in the die at step 604. A ram was added to fit the die, in achamber, and the chamber was evacuated and back filled with inert gas atstep 606. Heating was performed at step 608, while pressing, byconnecting a high amperage, e.g., 25,000 amp, alternating electricalcurrent power supply to the top ram and the bottom of the die which mayalso be a fitted ram. The electrical power applied raised thetemperature on the surfaces of the CP Ti disc that melted the aluminumin the powder mix. Heating was continued to cause the aluminum toexothermically react with the titanium while pressure was continued tobe applied in the range of approximately 10 MPa to 35 MPa (1.45 ksi-5ksi). During the liquid phase of the aluminum, a fully dense surfacelayer consisting of SiC particle in titanium aluminide that can includeTiAl, Ti₃Al or TiAl₃ which the TiAl is referred to as gamma titanium wasformed. Other alloying elements known to produce good gamma alloys suchas Nb, Cr, Mn, Si, etc. can be added to the Al—SiC powder mix. Themelting of the aluminum reacts with the CP Ti disc to form ametallurgical bonded surface layer to the CP Ti disc. With the appliedpressure the SiC particulate reinforced titanium aluminide is fullydense and free of porosity, a highly desirable feature for brake rotors.The entire heating process was quite rapid, taking only about 60seconds. The electrical current power supply was disconnected, and thedie assembly containing the coated brake rotors was blasted with coolinginert gas or air, depending on temperature, for about 5 minutes at step610.

The substrate can be CP Ti disc or any titanium alloy and heating can beperformed by pulsed or continuous electrical current, microwave,induction, radiation, etc. as an alternate to the resistant heatingbetween the rams. Also any ceramic particle or combination of ceramicparticles can be utilized and that other exothermic heating reactionswhich can be intermetallic formers other than aluminum can be used suchas Cu, Fe, Ni, Co. It is also recognized that the resistant heating canbe accomplished with any frequency current, direct current as well aspulsing the current.

EXAMPLE VIII

The same set-up as Example VII was utilized. When the resistive heatinghad brought the aluminum to its melting point, a high discharge of powerfrom additional power supplies or capacitor bank was induced and with aresidual atmosphere of argon or helium a plasma becomes generated thatinstantaneously raised the temperature of the disc and the surfacecoating. With pressure applied the aluminum reacted with the titaniumsurface that produced a fully dense pore free surface conversion layer.This process is often referred to as spark plasma sintering (SPS). Theplasma generated cleans the particle surfaces and enhances consolidationthat is well known in the art. If sufficient pulse of power is initiallyadded, no prior resistant or other form of heating is required thatshortens the overall consolidation time to produce the fully dense porefree surface/coating composite with the SiC.

The titanium can be any alloy, the ceramic particles can be singular ormixed to produce a custom surface property and that heat generatingreactions including intermetallics other than aluminum such as Cu, Fe,Ni, Co can be used and the ionizing gas can be other than argon orhelium to produce the plasma, and the process can be carried out atreduced pressure, ambient pressure or even positive pressure.

EXAMPLE IX

As yet another example, a vehicular brake rotor is formed using anintegrated one piece hub-disc. The cost of the brake discs becomesenabling to their use in automobiles. After a composite disc isfabricated as a sandwich of a composite layer on each side of amonolithic core or as a composite through the entire thickness of thedisc, a hub must be welded to the disc that permits mounting to thewheel assembly. The separate making of the hub and its welding to thedisc which may require a subsequent machining operation for any loss inparallelism due to the welding operation are all costly procedures. Analternative process that reduces cost is to utilize a casting, forging,or alternative process that results in a disc and hub produced in onepiece which is then coated with a layer of composite on the disc portionof the assembly. The integrated hub-disc one piece is placed in a dieset such as graphite that contains an outer ring with a solid graphitedisc for the base. A mixture of aluminum-silicon carbide (Al—SiC) powderis placed in the bottom of the die, on top of the solid graphite discpiece with an additional graphite plug for the hole on the bottom of thehub-disc opposite the side with the protruding hub. After the one piecehub-disc is placed in the die, an additional mixture of Al—SiC is placedon the disc portion of the one piece hub-disc followed by a die topconsisting of a hole to cover the hub. The loaded die is placed in achamber which is closed to the atmosphere. The residual atmosphere isexchanged via purging with an inert gas such as argon or nitrogen, orevacuated by applying a vacuum. The die is heated to a temperature thatthe Al alloy metal matrix composition melts but the one piece hub-discdoes not melt. Pressure is applied in the range of (1 to 8 MPa) to thedie that consolidates the Al—SiC alloy and metallurgically bonds to thecore one piece hub-disc. Pressure remains applied throughout the coolingcycle that produces a layer of Al—SiC alloy with a porosity of less than2%. The percent of SiC in the composite layer can be varied fromapproximately 25% to 65% and preferably 35% to 50%.

EXAMPLE X

As another example, the process of example XI was repeated, except theformed brake disc was subjected to heat treatment to increase hardnessand improve its mechanical properties. Preferably the heat treatmentcomprises a so-called T6 temper also known as solution heat treatingplus aging in which the sandwich composite is first allowed to coolnaturally. The sandwich composite is then heated to a temperature of510° C. in a high temperature oven. After 4 hours, the casting isremoved from the oven and quickly quenched. The casting is then moved toa low temperature oven and held at 170° C. for 12 hours and then aircooled

EXAMPLE XI

An additive manufacturing (AM) process can be utilized following theteachings, for example of our prior U.S. Pat. No. 8,203,095, thecontents of which are incorporated herein by reference, to form acomposite that is the full thickness of a rotor or as a surface reactionor cladding. Additive manufacturing consists of a CAD driven power beamsource that melts powder or wire in a continuously moving spot thatbuilds material layer-by-layer or transforms a surface that can build-upa clad or coating layer. The processing to produce a power beam includeelectron beam (e-beam), laser or plasma transferred arc (PTA) whereinthe work piece is one electrode and a torch head is the other electrodethat ionizes a gas such as argon or helium that forms the plasma beambetween the head and the part. A brake rotor can be built with any ofthe power beam processes by co-feeding the metal and ceramic particle tobuild layer-by-layer. For example, aluminum wire or powder can be fed tothe melt pool spot with a ceramic powder also co-fed to produce thecomposite. The CAD program can be set to build an entire rotor in thethickness desired at virtually any ceramic particle volume desired thatincludes that most desirable for brake rotors of 35-60%. The ceramic canbe varied in concentration at the start of the build and decreased untilthe final layer thickness is built and again added at a highconcentration to provide high wear resistance on each surface of therotor.

Any power beam process can be used to build the brake rotorlayer-by-layer in any desired ceramic particle loading includinggraduations to each surface in aluminum or its alloys, titanium or itsalloys, or other structural metals that can be used as rotors such asCu, Fe, Ni, Co, Zn.

In a specific case pure 1100 aluminum alloy powder can be used as thematrix and SiC particles as the ceramic filler. The PTA system can beutilized as the power beam source. A feed mixture of 1100 aluminumpowder and SiC powder can be used to produce a composite composition of40 volume % SiC in the aluminum matrix. The PTA system can be set for abuild rate of 5 lbs/hr to build the entire rotor in one hour since atypical rotor of the approximate size of 310 mm diameter with a 150 mmopen center hole weighs less than 5 pounds in an aluminum-SiC composite.

EXAMPLE XII

The PTA AM system can be utilized to apply an aluminum-SiC compositionto the surface of a CP Ti disc precut to size. The CAD system and powerto the PTA can be set to just melt a few mm of the surface of the discwith a powder feed mixture of 356 aluminum alloy and SiC powder toproduce a layer of titanium aluminide containing 50 volume % SiC. ThePTA beam power can be controlled to compensate for the exothermicreaction of the aluminum with the titanium so as not to melt too deep alayer of the CP Ti disc. This power beam processing can form the surfaceclad layer in base materials other than titanium. Base materials thatinclude aluminum and cast iron or steels can be used. If aluminum werethe base rotor material, titanium powder can be used to produce theexotherm to form the composite clad layer. Standard cast iron rotors canbe used to form surface layers of FeAl, Fe₃Al or FeAl₃ as well astitanium to form the titanium intermetallic TiFe₂ or TiFe.

Any power beam process can be used to melt the surface of the titaniumdisc that could be any titanium alloy. Intermetallics other thanaluminum such as Cu, Fe, Ni, Co, Zn could be used to form the cladreactive layer surface along with any single or mixed compositionceramic particle as well as base alloys of cast iron or steels ontowhich the intermetallic-ceramic particle alloy may be formed.

EXAMPLE XIII

A friction stir welding system can be modified to provide a powder feedinto the rotating head that is controlled with a CAD system thatprovides controlled downward pressure on the rotating head as well ascontrolled speed forward in a tool path. A mixture of 50 volume % SiCpowder and aluminum powder can be fed to the rotating head that hassufficient downward pressure that the friction causes the surface of thetitanium substrate to melt with the aluminum in the feed, generatingexothermic heat contributing to the melting under the friction stirprocess rotating head that encapsulates the SiC particles in the moltenlayer that on solidification as the rotating head moves forward in itsCAD guided tool path, a surface of titanium aluminide containing SiCparticulate is produced free of porosity.

To speed the forward movement of the rotating head with controlleddownward pressure, in front of the head, additional heat can be appliedsuch as a flame torch, laser beam, electron beam, plasma transferred arc(PTA) beam, induction coupling, or a focused radiation beam such as froma tungsten-quartz lamp. The rotational speed and downward pressuresrotating head can be somewhat relieved to generate a molten path on thetitanium rotor with the additional heat applied in front of the FSPprocessing. With the CAD control of the FSP the composite layer formedcan be over the entire surface of a CP Ti disc precut to size for abrake rotor, or just in a path that is the width of the brake pad.

The thickness of the titanium aluminide-SiC layer can be controlled bythe depth of the melt generated by the rotating head speed and itsdownward pressure, the exotherm from the Al—Ti generated heat as well asany auxiliary heat supplied in front of the forward moving rotatinghead. Surface layer thickness can be as little as approximately 0.25 mmup to approximately 5 mm. It is possible with a very hard and strongrotating head with sufficient downward pressure and rotating speed tomelt depths of 10 or more mm. Such a friction stir capability permitsthe entire cross-section of a brake rotor to be produced as a compositeby friction stir processing (FSP) which is possible in a single pass ormultiple passes analogous to layer-by-layer as delineated in additivemanufacturing.

While this example is given for a CP Ti base rotor, other base metalssuch as aluminum, cast iron, steels, copper, and zinc can be utilized toproduce a composite with single or mixed ceramic particles by frictionstir processing (FSP).

The foregoing description discusses implementations and preferredembodiments of the present invention, which may be changed or modifiedwithout departing from the scope of the present invention as defined inthe claims. The examples listed in parentheses may be alternative orcombined in any manner. The invention includes any practical combinationof the structures and methods disclosed. As used in the specification,the words ‘process’ and ‘method’ are synonymous. As used in thespecification and claims, the words ‘having’ and ‘including’ in allgrammatical variants are open-ended and synonymous with ‘comprising’ andits grammatical variants. While for the sake of clarity of descriptionseveral specific implementations and embodiments of the invention havebeen described, the scope of the invention is intended to be measured bythe claims as set forth below.

What is claimed is: 1: A system for forming a layer bonded to a surfaceof a solid substrate, the system comprising: a. a die having a base anda removable top, for enclosing the substrate in a form of a hub-discassembly and a powder mixture consisting of a powder metal and a ceramicparticulate, wherein the hub-disc assembly melts at a first temperature,and wherein the metal or metal alloy component of the powder mixture isdifferent from the metal or metal alloy forming the substrate, andconsists of a metal or metal alloy which melts at a second temperatureless than the first temperature; b. a heating subsystem configured torelease energy from a provided source for heating contents of the die;c. a mechanical subsystem configured to apply a force against the diecontents; and d. a controller coupled to control the heating subsystemand the mechanical subsystem, the controller configured to enable anexothermic reaction between the surface of the substrate and the powderby heating the contents; and to enable densification of the contents byapplying the force for a period that includes time before the contentshave cooled to an ambient temperature. 2: The system of claim 1, furthercomprising: a vacuum subsystem adapted to remove gas from the contentsof the die via the optional vent. 3: The system of claim 1, wherein thesubstrate comprises a vented hub-disc assembly. 4: The system of claim1, wherein the powder mixture is press consolidated and the consolidatedpowder mixture is placed on the substrate in the die and treated to bondthe aluminum in the consolidated powder mixture directly to thesubstrate without an intervening layer. 5: The system of claim 1 whereinthe first temperature corresponds to the melting point of an element ofthe group consisting of aluminum, cobalt, copper, iron, nickel,titanium, vanadium, and zinc, or of an alloy comprising one or moreelements of the group consisting of aluminum, cobalt, copper, iron,nickel, titanium, vanadium, zinc, chromium, magnesium, manganese,niobium, and silicon. 6: The system of claim 1, wherein the powdermixture consists of aluminum or an aluminum alloy, and a ceramicparticulate. 7: The system of claim 1, wherein: a. the hub-disc assemblycomprises aluminum or an aluminum alloy, titanium or a titanium alloy,or an iron/steel alloy; and b. the powder metal consists of aluminum oran alloy of aluminum and silicon, or aluminum, silicon and magnesium, oraluminum and titanium, or aluminum, silicon and titanium, or aprealloyed powder selected from Ti—Al and Ti—Al—Cr—Nb. 8: The system ofclaim 6, wherein the ceramic particulate comprises silicon carbide. 9:The system of claim 1, wherein from 25% to 65% of an exterior surface ofthe layer formed comprises ceramic particulate. 10: The system of claim1, wherein the layer formed has a porosity selected from the groupconsisting of less than 2% by volume, less than 1% by volume, and lessthan 0.25% by volume. 11: The system of claim 1, wherein the layerformed and the hub-disc assembly comprise a product having a porosityselected from the group consisting of less than 2% by volume, less than1% by volume, and less than 0.25% by volume. 12: The system of claim 1,wherein the powder mixture of the surface layer is further characterizedby one or more of the following features: a. the powder mixture isbinder free; b. the powder mixture consists of about 61% by weightaluminum silicon alloy, and about 39% by weight silicon carbide; c. thepowder mixture consists of about 59.5% by weight aluminum silicon alloy,about 38% by weight silicon carbide, and about 2.5% by weight titanium;d. the powder metal comprises aluminum silicon alloy having adistribution of particle sizes defined by D10 about 5 microns, D50 about15 microns, and D90 about 38 microns; e. the powder mixture comprisessilicon carbide having particle size from about 10 microns to 20microns; f. the powder metal comprises titanium having a particle sizeof 100 microns or less; g. the powder metal comprises aluminum alloy 356or aluminum alloy 432; h. the powder metal comprises an aluminum alloythat melts at or below 600° C.; i. the powder metal comprises aluminumwith 1 wt % to 20 wt % silicon alloy; j. the powder metal comprisesaluminum with 1 wt % to 12 wt % silicon alloy; and k. the powder mixturecomprises silicon carbide particles and 95% of the silicon carbideparticles have particle size below about 50 microns. 13: The system ofclaim 1, wherein the hub-disc assembly consists essentially of aluminum1100 or aluminum alloy
 5052. 14: The system of claim 1, wherein thepowder mixture is spread against one or more surfaces of the substrate.15: The system of claim 1, wherein the powder mixture is spread againstone or more surfaces of the hub-disc assembly at a thickness selectedfrom the group consisting of from 0.001 inch to 0.25 inch, 0.60 to 0.125inch, 0.40 to 0.080 inch, and about 0.050 inch. 16: The system of claim1, wherein a quantity of liquid melt is reduced by the formation of areaction product in a solid phase. 17: The system of claim 1, wherein 35volume % or more of an exterior surface of the layer comprises ceramicparticulate. 18: The system of claim 1, wherein the substrate consistsessentially of an aluminum alloy that melts above 580° C. 19: The systemof claim 1, wherein the powder metal comprises titanium having aparticle size of about 44 microns.